This non-provisional application claims priority under 35 U.S.C. §119(a) on Korean Patent Application No. 2004-96833 filed on Nov. 24, 2004, which is herein expressly incorporated by reference.
1. Field of the Invention
The embodiments of the present invention relate to a method for surface treatment of nickel nanoparticles using an organic solution. More particularly, the embodiments of the present invention relate to a method for surface treatment of nickel nanoparticles comprising: 1) dispersing nickel nanoparticles in a reductive organic solvent to obtain a dispersion; 2) heating the dispersion to increase a surface homogeneity of the nanoparticles; 3) separating the nanoparticles from the dispersion; and 4) washing and drying the nanoparticles.
2. Description of the Related Art
A multi-layer ceramic capacitor (MLCC) is prepared by laminating a multiplicity of dielectric thin film layers and a multiplicity of internal electrodes. The MLCC having such a structure exerts large capacitance even with a small volume and thus is widely used in electronic instruments such as computers and mobile communication instruments, for example.
As materials for internal electrodes of the MLCC, Ag—Pd alloys have been used. Since Ag—Pd alloys can be sintered in the air, they are easily applicable to MLCC manufacturing, but are disadvantageously expensive. Therefore, in order to reduce MLCC production costs, since the late 1990's, a great deal of efforts have been made to replace Ag—Pd alloys with relatively cheap nickel, as an internal electrode material. Internal nickel electrodes of the MLCC are formed from a conductive paste containing nickel metal particles.
Processes for preparing nickel metal particles are broadly divided into a vapor phase method and a wet chemical method. The vapor phase method is widely used due to the fact that shapes of nickel metal particles and impurities are relatively easily controlled, but is disadvantageous from the standpoints of particle micronization and large-scale production. In contrast, the wet chemical method is advantageous in that large-scale production is easily implemented and initial investment costs and process costs are inexpensive.
The wet chemical method may be sub-divided into two types. One is a method using nickel hydroxide as a starting material that is converted into nickel metal particles. The other is a method using nickel precursor materials other than nickel hydroxide, such as nickel salts and nickel oxides, as a starting material that is converted into nickel metal particles.
The first method is advantageous in that a manufacturing process is relatively simple, but also suffers from disadvantages in that the starting material, nickel hydroxide, is expensive and controlling particle size of nickel metal particles is not easy.
The second method is disadvantageous due to relatively complex processes. But it has an advantage in that inexpensive nickel precursor materials such as nickel sulfate, nickel chloride and nickel acetate may be used as the starting material, and it is relatively easy to control the particle size ranging from several tens of nm to several hundreds of nm.
As patents relating to the wet chemical method, reference may be made to U.S. Pat. Nos. 4,539,041 and 6,120,576.
U.S. Pat. No. 4,539,041 proposes a method of obtaining metal powder, involving dispersing gold, palladium, platinum, iridium, osmium, copper, silver, nickel, cobalt, lead and cadmium in the form of an oxide, hydroxide or salt thereof, in a liquid polyol as a reducing agent and thereafter heating them.
U.S. Pat. No. 6,120,576 proposes a method for preparing nickel metal powder comprising the steps of mixing an aqueous sodium hydroxide solution with an aqueous solution of nickel sulfate to form nickel hydroxide; reducing the resulting nickel hydroxide with hydrazine to produce nickel; and recovering nickel thus produced.
In the above-mentioned methods, an alkali is added in order to effect conversion of the nickel precursor compound into nickel hydroxide. As the alkali added, sodium hydroxide, potassium hydroxide or the like is conventionally used. In order to synthesize nickel fine powder, the desired reaction should be performed in the presence of a strong base, depending on reaction conditions, and such environment allows for formation of Ni(OH)2 on a nickel surface and the thus formed surface Ni(OH)2 grows to plate-like or needle-like particles depending on reaction conditions.
In order to provide a high capacitance MLCC, packing density of particles should be high. However, such secondary particles referred above, during an MLCC process, contribute to lower density in packing and occurrence of shorts. Therefore, a smoothness of the particle surface is required.
Japanese Patent Publication Laid-open No. Hei 4-235201 discloses a method involving adding metal powder to stearic acid dissolved in an organic solvent and removing the organic solvent via evaporation from the mixture so as to control tap density of powder. Japanese Patent Publication Laid-open No. Hei 12-345202 discloses surface treatment of nickel microparticles with decanoic acid, caprylic acid, palmitic acid, linolic acid, linoleic acid, oleic acid, stearic acid or the like, so as to effect high tap density, high packing density of nickel fine powder in a paste, and, at the same time, in particular, thin application thickness of the paste to obtain internal electrodes of a laminated ceramic condenser at a predetermined thickness. Japanese Patent Publication Laid-open No. Hei 15-129105 discloses a method involving first surface treatment of nickel powder with an aqueous solution containing water-soluble fatty acid salts and then heating the mixture to volatilize the solvent. However, the above-mentioned methods may entail occurrence of particle-particle aggregation upon heating and thus there still remains a need for a method capable of treating surfaces of nickel particles by a simplified process.